Ever since the English scientist Stephen Hales first measured the blood pressure by observing the blood rise in a tube inserted in an artery of a horse in 1733, scientists and physicians have sought better ways to measure blood pressure in people.
An instrument in common use for indirectly measuring blood pressure is a sphygmomanometer, which comprises an inflatable cuff which wraps around the upper arm above the elbow, a rubber bulb to inflate the cuff, and a device to measure the levels of pressure. It is well known that if the cuff is inflated to above systolic pressure, then slowly decompressed, oscillations corresponding to the heart rate will appear in the cuff pressure beginning somewhat above systolic pressure. These oscillations typically reach a maximum amplitude and then diminish until they are lost. The French physiologist, E. J. Marey, who discovered this phenomenon in 1876, reasoned that the peak amplitude of oscillation occurred close to mean arterial pressure. This hypothesis was confirmed by later investigators, and various methods of blood pressure determination based on the "oscillometric principle" were subsequently developed.
In 1905, Dr. N. S. Korotkoff proposed an auscultatory method of determining blood pressure. In this method, an arm cuff is inflated until it stops the circulation of blood beyond the cuff. Thereafter, a stethoscope is used to listen to the artery just distal to the cuff. Korotkoff hypothesized that the first sounds heard when the arm cuff is slowly deflated correspond to maximum pressure, whereas minimum pressure occurs when the sounds disappear. Later laboratory and clinical studies confirmed the accuracy of the auscultatory method, which eventually became universally adopted in clinical medicine.
The above techniques have heretofore been considered to provide insufficiently precise measurements for adequate management of cardiac pressures in critically ill patients. It has also not been possible to non-invasively determine left ventricular preload, which heretofore has been determined invasively by measuring the mean left atrial pressure or the pulmonary capillary wedge pressure.
In 1953, Lategola and Rahn demonstrated the efficacy of a flow-directed pulmonary artery catheter for the direct measurement of pulmonary artery pressure. Lategola and Rahn, A Self-Guiding Catheter for Cardiac and Pulmonary Arterial Catheterization and Occlusion, 84 Proc. Soc. Exp. Biol. Med. 667-668 (1953). In 1970, Swan, Ganz, and associates reported use of a flow-directed catheter in humans and further refined it for clinical use and for the direct measurement of pulmonary capillary wedge pressure. Swan, Ganz, Forrester, Marcus, Diamond, and Chonette, Catheterization of the Heart in Man With Use of a Flow-Directed Balloon-Tipped Catheter, 283:9 The New England J. Med. 447 (1970). At present, this catheter is an invaluable aid in the management of critically ill patients with pulmonary and cardiac disease, and the pulmonary wedge pressure (as an estimation of mean left atrial pressure or left ventricular filling pressure, commonly referred to as preload) is the standard of reference for intravascular volume management.
Numerous potential indications for pulmonary artery catheterization are now accepted. For example, catheterization is widely used in the evaluation and management of patients with acute myocardial infarction, for patients in shock, in the recognition of hypovolemia, in the treatment of patients suffering respiratory failure with persistent hypoxemia, and in patients with congestive heart failure. Catheterization is especially useful in assessing cardiac function in surgical patients, both pre-, intra-, and postoperatively. Since 1970, the ability to measure pulmonary capillary wedge pressure and cardiac output with the flow-directed catheter has resulted in the development of bedside hemodynamic monitoring, a procedure now performed daily in most hospitals in the United States. J. M. Gore et al., Handbook of Hemodynamic Monitoring, 3 (1985). Since the introduction of the Swan-Ganz catheter in 1970, it is reported that several million pulmonary catheters have been placed in patients with acute myocardial infarction. Gore et al., 92:4 Chest, 712 (October 1987).
Despite the widespread use of the pulmonary artery flow-directed catheter, the procedure is not without drawbacks. Complications that may arise from use of the catheter include pulmonary artery thrombosis or embolus, knotting of the catheter, rupture of the balloon and/or of a pulmonary artery, pulmonary hemorrhage, pneumothorax, hemothorax, right atrial thrombosis, sepsis, internal jugular stenosis or thrombosis, atrial and ventricular arrhythmias, electromechanical dissociation, right-sided endocardial lesions, and right-sided endocardial infection. Robin, The Cult of the Swan-Ganz Catheter, Overuse and Abuse of Pulmonary Flow Catheters, 103:3 Annals of Internal Medicine 445 (September 1985). In recent years, the safety and efficacy of pulmonary artery catheterization has become a subject of increased scrutiny and concern. One study suggests that flow-directed pulmonary artery catheterization may predispose patients to the development of right-sided endocarditis. Rowley, Clubb, Smith, and Cabin, Right-Sided Infective Endocarditis as a Consequence of Flow-Directed Pulmonary-Artery Catheterization, 311:18 The New England J. Med. 1152 (Nov. 1, 1984). The medical literature abounds with articles addressing the numerous medical complications associated with pulmonary artery catheterization. See, e.g., Murray, Complications of Invasive Monitoring, 15:2 Medical Instrumentation 85 at p. 89, March-April 1981, which lists various references related thereto. Perhaps the most serious allegation to date is that complications associated with the use of the pulmonary artery catheter in patients with acute myocardial infarction have resulted in an unusually and unacceptably high mortality rate. Robin, Death by Pulmonary Artery Flow-Directed Catheter. Time for a Moratorium? (editorial), 92:4 Chest 727 (October 1987).
In addition to the safety concerns, there is a relatively high monetary cost of critical care invasive monitoring, which cost may be minimized by the availability of a non-invasive procedure where indicated. Thus, a need has existed for a non-invasive and less costly improved method for accurately measuring blood pressure in the left atrium in people.
Invasive hemodynamic measurement nevertheless remains an important and feasible adjunct to clinical practice. Successful monitoring permits accurate determination of the state of the diseased heart and provides guidance for treatment and intervention to alter the course of a variety of diseases. It is recognized that modern Swan-Ganz catheters allow for the measurement of cardiac output, oxygen consumption, continuous mixed venous oxygen saturation, and cardiac pacemaking, and that many critically ill patients will require this degree of sophisticated monitoring. Nevertheless, given the knowledge of mean left atrial pressure and left atrial transmural pressure alone, there are numerous patients who could be safely managed in intermediate care units or on regular nursing floors. Certain patients undergoing general anesthesia could also benefit from less invasive monitoring of mean left atrial pressures. Furthermore, a less invasive technique for the measurement of mean left atrial pressure could be used to rationally screen patients to determine whether or not they would benefit from Swan-Ganz catheterization; otherwise, monitoring of mean left atrial pressure by such a less invasive technique may suffice to manage the patient outside the intensive care setting.
Thus, a long-felt need exists for a non-invasive method to accurately determine mean left atrial pressure. This is a ASK primary underlying objective of the present invention.
An esophageal catheter with a balloon having an inflated length and diameter of 3.1 cm. and positioned adjacent the left atrium has previously been used in an attempt to provide the shape of the curve of left atrial pressure. See Gordon et al, Left Atrial, "Pulmonary Capillary", and Esophageal Balloon Pressure Tracings in Mitral Valve Disease, British Heart J., 18: 327-340, 1956.
In order to record left atrial events, Gordon et al suggests, at page 330, that the esophageal balloon to be positioned adjacent the left atrium must be relatively small, "otherwise the tracings will be distorted by pressure or volume changes taking place at other than the desired left atrial level" and that it was "usually necessary to suspend respiration while the records were being made."
However, Gordon did not provide pressure measurement and, indeed, stated that his system was incapable of obtaining left atrial pressure values. Thus, Gordon et al states, at page 330, that "no attempt was made to measure absolute pressures from these tracings, as the amplitude of the pressure pulse is a function of the elasticity of the system, the amount of fluid in the balloon and the initial pressure within it, as well as the intra-atrial pressure." As again indicated at page 338 of Gordon et al, one of the drawbacks of the Gordon et al system is the inability to obtain absolute left atrial pressure values. That was more than 30 years ago.
A concern when attempting to pick up left atrial pressure waves using balloon-tipped esophageal catheters is the problem of insuring that the balloon is properly positioned behind the left atrium. In connection with the placing of electrodes for transesophageal heart pacing, it has been suggested that a positioning balloon may be inserted on the distal end of an esophageal catheter to anchor the catheter in the stomach. Since the distance between the left atrium and the stomach (gastroesophageal junction) is relatively constant in an adult, the pacing electrodes could then be affixed to the catheter at this distance proximal to the stomach balloon. See Andersen et al, Trans-Esophaaeal Pacing, PACE, Vol. 4, July-August, 1983, pp. 674-679. However, this process is not suitable for use with non-adults since the gastroesophageal junction to left atrial distance will not be constant but will vary for neonates and children. It has also been suggested, in connection with observing the esophageal pulse in mitral valve disease, that an electrode may be used to position an esophageal balloon behind the left atrium by attaching it to the catheter just above the balloon to measure the esophageal electrocardiogram from behind the left atrium. See Zoob, The Oesophageal Pulse in Mitral Valve Disease, British Heart J., Vol. 16, 1954, pp. 39-48. Also see Brown, A Study of the Esophageal Lead in Clinical Electrocardiography, American Heart J., Vol. 12, No. 1, July, 1936, pp. 1-45; and Oblath and Karpman, The Normal Esophageal Lead Electrocardiogram, American Heart J., Vol. 41, 1951, pp. 369-381.
While mean left atrial pressure is considered important, pulmonary venous transmural pressure or mean left atrial transmural pressure (hereinafter called "transmural pressure") is considered to be a more clinically useful physiologic value because it allows physicians to more precisely determine when a patient could go into pulmonary edema from heart failure or volume overload, and it also allows an assessment of the effect of positive end expiratory pressure with ventilated patients. For the purposes of this specification and the claims, transmural pressure is equal to the difference between mean left atrial pressure and the intrapleural pressure. The tissue pressure in the chest (mediastinum) reflects the intrapleural pressure, which is usually sub-atmospheric at end expiration during regular breathing while sitting erect. However, the transmural pressure has not been commonly used by physicians because it has heretofore not been readily obtainable and available.
It is an object of the present invention to non-invasively obtain quantitative pressure measurements to readily determine a person's mean left atrial pressure as well as the transmural pressure safely, accurately, and reliably. As used herein and in the claims, the terms "transmural pressure" and "pulmonary venous transmural pressure" are meant to refer to the mean left atrial transmural pressure.
It is another object of the present invention to obtain such measurements economically and easily.
It is a further object of the present invention to provide a method for determining a person's mean left atrial and transmural pressures which may be administered by a non-physician.
It is still another object of the present invention to non-invasively obtain quantitative pressure measurements to readily determine other pressures associated with the left atrium safely, accurately, and reliably.